There are current two types of direct gas-fired heating and ventilating systems used in industrial and commercial buildings and the like to meet the needs of make-up air for that which is exhausted from the building. Basic to both systems is the supplying of heated air into a building by passing fresh outside air directly over a gas flame with the burner utilizing the kinetic energy of the air flow to complete combustion without adverse by-products prior to delivery into the building space. In one type of system, 100 percent outside air is supplied to the gas flame and thence in a thus heated condition into the building to replace that amount of air which is exhausted. Moreover, in such a system, it is also possible to bring in an excess of make-up air over that exhausted to ensure a positive pressure in the area to prevent outside air infiltration and cold drafts, down or backdrafts in heating equipment flues, ventilators and stacks, and to compensate for reduced exhaust fan air volumes that could result in inadequate removal of possible contaminants. Normally, the exhaust system in this type of arrangement is interlocked with a make-up air system so that both units are in operation at the same time. During operation of this type of direct gas fired system, air flow is continuous with the discharge of air normally regulated either by modulating the gas flow or by electronic gas flame modulation. In either event, each of these control systems allows the burner to operate at any point on its turn-down range to provide a constant heater air discharge temperature.
In the other type of conventional direct gas-fired heating and ventilating system, there is incorporated a return air damper downstream of the direct gas-fired burner which provides an option of 50 percent fresh air and 50 percent return air, or 100 percent fresh air and no return air. The latter mode is generally recommended for multiple exhaust fan installations and other applications with variable exhaust air requirements recognizing that less fuel is expended to heat less make-up air. In a system with multiple exhaust fan applications, the 100 percent fresh air damper position is established when all the exhaust fans are operating. On the other hand with only one fan in operation, the unit is typically switched to handle 50 percent make-up air and 50 percent return air. This kind of system lends itself to those ventilation applications incorporating supplemental space heating with make-up air requirements. In such an application, the total air volume circulated need not be 100 percent make-up air but can be a mixture of 50 percent fresh air and 50 percent return air. However, since the heat output of the burner at the 50 percent fresh air and 50 percent return air condition must not be allowed to normally exceed one-half that of the 100 percent fresh air condition, the unit must be capable of providing needed supplementary heat at the reduced firing rate. And thus the later system has the same modulating discharge air control as in the fresh mentioned system and in addition has a fresh air/return air control.
While the above systems have proven generally satisfactory, they are at best a compromise as the total amount of air being handled and the amount of outside air being discharged by the air handling equipment must be held constant to ensure proper conditions for good combustion at the gas flame. A significantly better system would be where both the outside air and the return air could be modulated to satisfy any air conditioning and ventilation demand while heating or cooling.